By parallel implementation, digital signal processing semiconductor devices have extended the operating speed to tens of gigabit per second. With this development, a digital coherent optical communication that enables multilevel transmission using the digital signal processing is studied as a long haul and large capacity communication technique and is making progress.
In the digital coherent optical communication, both intensity and phase of an optical signal are manipulated with a digital circuit. As a result, the chromatic dispersion and the fast-fluctuating polarization mode dispersion (PMD) in an optical fiber are easily compensated, and high sensitivity due to the coherent receiving is also expected.
Related art is disclosed in International Publication Pamphlet No. WO 2009124961, Rasumussen, Hoshida, and Nakashima, “Digital Coherent Receiver Technology for 100-Gbps Optical Transport Systems, FUJITSU. 60, 5, pp. 476-483, September 2009, and the like.